Outdoor radio frequency position location systems have grown from specialized time-of-flight (“TOF”) or return time-of-flight systems (“RTOF”) such as long range aid to navigation (“LORAN”) transmitters used by mariners and pilots to the present-day global positioning system (“GPS”) receivers found in many vehicles, smartphones, and similar handheld electronic devices. While such outdoor-use positioning systems have revolutionized the way in which people navigate and find their location on the surface of the Earth, an inherent weakness in such systems is their inability to accurately determine a user's location when in an enclosure or in an indoor or covered location. In particular, such conditions include instances where a direct line of sight to one or more position location transmitters such as a cellular or radio tower or satellite, is obstructed or unavailable. In such situations, position location information either is not reported or is reported with a significant degree of positional uncertainty.
Indoor position location is valuable in many applications that include, but are not limited to: manufacturing (e.g., tracking an article or a collection of articles through a manufacturing process), warehousing (e.g., determining the location of one or more items), merchandising (e.g., finding consumer items in a retail environment or shopping mall) and security (e.g., finding an escape route from a building). Indoor position location technologies include systems that are able to provide absolute location data (e.g., position location data relative to an absolute scale such as latitude and longitude) or relative location data (e.g., position location data relative to a non-absolute scale such as location within a building or location relative to one or more transmitters. In many instances, such indoor position location systems make use of trilateration or multilateration techniques including time-of-arrival (“TOA”), TOF, or RTOF in which the location of a mobile receiver is determined based on the time required for a signal to propagate between one or more fixed transmitters and the mobile receiver. Such systems rely upon the presence of a highly accurate timekeeper in at least one of either the fixed transmitter or mobile receiver and/or the transmission of time stamp generated by a highly accurate timekeeper between the transmitter and the receiver to accurately determine the time shift of the signal and hence the distance between the fixed transmitter and the mobile receiver. Such time-of-flight systems require a significant investment in fixed transmitter infrastructure and/or mobile receivers having highly accurate timing or clocking systems to provide the necessary resolution enabling an accurate positional fix. Additionally, signal reflections and shadow areas within the environment may adversely impact the accuracy of time-of-flight systems.
Other methods for determining an indoor location exist. One such method includes angulation where the angles of arrival of a number of signals generated by a respective number of fixed transmitters in known locations are detected using a mobile device. By combining angle of arrival data acquired from at least two of the fixed transmitters, a two (or three) dimensional position corresponding to the location of the mobile device may be determined. Such systems however rely upon the use of costly directional antennas or antenna arrays coupled to the mobile device to accurately determine the angle of arrival of the incoming signals. Additionally, the performance and accuracy of such angle of arrival systems is compromised in environments where signal reflections and shadows exist.
Given the various compromises, costs, and accuracy issues surrounding available indoor position location systems, what is needed is an accurate yet flexible low cost indoor positioning system.